control the rotation of the particle instance to face to camera
Thanks CG Boost: https://www.youtube.com/watch?v=aLVodLos41Q

Here is a summary:


Create an Empty that you parent to your camera but placed far away behind it on the local Z axis. This will be used to lock the particle source orientation that will "look" at that general direction
 
place the source particle object somewhere in the middle of where most particles will be (in the middle of the forest) 
Add a Track to Constraint to the source particle object, use the empty behind the camera as target
Add a Limit Rotation constraint to the same object to keep it strait.
 


This way, generated particles should all face the general direction of the camera as long as the empty is far enough behind it.



Also if you have trouble with your source object in the middle of your scene, just put it on a difference scene along with a linked duplicate of the camera.